Improved desolventizer toaster with vapor recycle

ABSTRACT

An improved desolventizer toaster and methods are provided wherein a solvent vapor recycle system may be employed to recycle solvent vapor throughout a portion of the desolventizer portion of the apparatus housing. In other embodiments of the invention, a water vapor recycle system may be employed to recycle water vapor throughout a portion of the toaster portion of the apparatus housing. The solvent vapor recycle system may comprise an appropriate meal dust separator, blower and superheater. The water vapor recycle system may comprise an appropriate ejector or blower and superheater. Water vapor recovered in the solvent extraction process may be used as a water vapor feed to the heated water vapor recycling system.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of U.S. Provisional PatentApplication Ser. No. 61/217,921 filed Jun. 5, 2009.

FIELD OF INVENTION

The invention relates to an improved apparatus and process forsimultaneously evaporating residual solvent from spent oilseed materialwhile subjecting such spent oilseed material to conditions that createoilseed meal with improved protein digestibility, while maintaining lowresidual solvent and adequate inactivation of anti-nutritional factors.

BACKGROUND OF THE INVENTION

Oilseed crushing, the production of vegetable oil and animal feed fromoilseeds, nearly always entails a solvent extraction step in whichparticulate, oilseed material is contacted with an organic solvent suchas but not limited to hexane and its isomers to separate the liquidfraction from the solid fraction. The resultant liquid fraction is knownin the trade as “miscella”, a solution of vegetable oil in the organicsolvent, and the resultant solid fraction is known in the trade as“spent material”, the extraction residue soaked with organic solvent.After evaporating the solvent from the miscella, the crude vegetable oilis further processed to produce edible oil products. After evaporatingthe organic solvent present in the spent material, an oilseed meal isproduced that is primarily used as a high protein animal feedingredient.

The spent material from the solvent extraction process is generallysoaked with twenty-five to thirty-five percent (25-35%) solvent byweight. The downstream process of evaporating the solvent from saidspent material is known in the trade as “desolventizing”, whose primarypurpose is to remove the organic solvent to as low a residual solventcontent as possible for both feed safety and environmental purposes.

Most oilseeds contain anti-nutritional factors (ANF) that prevent keyamino acids in oilseed meals from being digested by mono-gastric animalssuch as poultry and swine. Said ANF generally consist of heat labilepeptides such as the trypsin inhibitors present in soybeans.Accordingly, heating the oilseed meal to a sufficiently high temperaturewith adequate moisture present for a sufficiently long periodinactivates these ANF by denaturing said peptides. The term used in thetrade for this process is “toasting”.

Toasting the meal simultaneously denatures the protein present in themeal. Denaturing the protein lowers the availability of the protein tobe digested by mono-gastric animals such as poultry and swine, whichtherefore lowers the feed value of the meal in these markets. The degreeof protein digestibility is often characterized in the trade by theprotein digestibility index (PDI) expressed in percent. Higher PDI meanshigher availability of the protein to be digested. The majority ofsoybean meal in the trade today has a PDI in the range of twenty tothirty-five percent (20-35%).

The prior art, as exemplified by U.S. Pat. No. 5,992,050, employsequipment whereby spent material is treated in an apparatus that isknown in the trade as a “desolventizer-toaster” (DT). The solventcontent of the spent material is generally twenty-five to thirty-fivepercent (25-35%) by weight, with a moisture content of ten to twelvepercent (10-12%) by weight and a temperature of 50-60° C.

The spent material continuously enters the DT vessel and drops onto atray inside, known in the trade as a “pre-desolventizing” (PD) tray,which is an indirect steam-heated horizontal disc with a surfacetemperature typically 135-155° C. A shallow layer of spent material isstirred above the surface of the PD tray by stirring blades extendingfrom a rotating centrally located shaft. Upon contacting the PD traysurface, the temperature of the spent material is increased to theboiling range of the solvent, typically 60-70° C., where the organicsolvent begins to evaporate. A typical DT has two to four PD traysstacked vertically in series, where approximately ten to twenty percent(10-20%) of the initial solvent is evaporated, while the temperatureremains 50-60° C. and the moisture remains relatively constant at ten totwelve percent (10-12%).

After the spent material exits the PD trays, the spent materialcontinuously enters the next chamber of the DT vessel. The spentmaterial drops onto a tray inside, known in the trade as a“counter-current” (CC) tray, which is an indirect steam-heatedhorizontal disc with a surface temperature typically 135-155° C. thathas special apertures therein that allow ascending superheated watervapor to rise through. A deep layer of spent material is stirred abovethe surface of the CC tray by stirring blades extending from a rotatingcentrally located shaft. Soon after falling to the material surface ofspent material supported above the CC tray, rising superheated watervapor from below condenses into the spent material layer providinglatent heat to evaporate the majority of the remaining solvent from thespent material. When the spent material exits this first CC tray it willtypically contain less than one percent residual solvent by weight witha moisture content of eighteen to twenty-two percent (18-22%) by weightand a temperature of 95-100° C. These moisture and temperatureconditions allow for destruction of ANF and reduction of proteindigestibility in the spent material.

A typical DT has two to four CC trays stacked vertically in series.After the first CC tray, the temperature and moisture remain relativelyconstant. The primary purpose of the remaining CC trays is to allow timefor further stripping of the remaining traces of residual solvent byrising superheated water vapor and time for adequate destruction of theANF. Reduction of the protein digestibility of the spent materialcontinues on the remaining CC trays.

After the spent material exits the final CC tray, the spent materialcontinuously enters the next chamber of the DT vessel. The spentmaterial drops onto a tray inside, known in the trade as a “sparge” (SP)tray, which is a hollow horizontal disc with perforated upper surfacefor even distribution of superheated water vapor into the DT. A deeplayer of spent material is stirred above the surface of the SP tray bystirring blades extending from a rotating centrally located shaft. Theevenly distributed superheated water vapor rises through the spentmaterial to achieve efficient removal of the remaining residual solventto acceptable levels. Final destruction of ANF and reduction of proteindigestibility also occur on the SP tray. When the desolventized, toastedmeal exits the DT vessel it will typically contain less than 0.05%solvent by weight with a moisture content of eighteen to twenty-twopercent (18-22%) by weight and a temperature of 105 to 110° C. The ANFof the desolventized, toasted meal as measured by urease delta pH istypically less than 0.15 and the protein digestibility as measured byPDI is typically 20-30%.

The solvent evaporated from the spent material along with an equilibriumquantity of water vapor typically exits the upper roof of the DT vessel.This vapor stream is generally 67-75° C. temperature and is composed of90-95% solvent vapor in equilibrium with 5-10% of water vapor. Thisvapor stream often contains traces of meal dust and therefore passesthrough a vapor scrubbing apparatus before passing on to heat recovery.

In summary, it is known in the trade that the DT operates at or nearatmospheric pressure and therefore the temperature of the spent materialreaches a temperature just above the boiling point of water, 105-110° C.It has been found that with a typical DT apparatus and process that theamount of superheated water vapor condensation required to evaporate theremaining solvent in the spent material after the PD trays typicallyincreases the moisture of the spent material to 20-22% by weight. Italso been found that to achieve a residual solvent content in thedesolventized, toasted meal at the exit of the DT of less than 500 partsper million (ppm), and preferably less than 250 ppm solvent by weight,that the spent material must remain in the CC and SP trays of the DTcollectively for 20 minutes or greater with a minimum rising superheatedwater vapor flux rate of 300 kg/hr water vapor per m² tray surface area.It is further known that the combination of 20-30 minutes residence timeat 105-110° C. and 20-22% moisture produces a meal with a proteindigestibility as measured by PDI of 20-30%. This represents the presentstate-of-the-art apparatus, process and results in the trade.

Accordingly, there is a need in the art to provide a DT process andapparatus wherein the resultant desolventized and toasted spent materialmeets the industry requirements for residual solvent content anddestruction of ANF, while simultaneously providing a meal product withsignificantly high protein digestibility as measured by PDI.

SUMMARY OF THE INVENTION

It has been discovered by analytical testing that if the spent materialis subjected to a temperature of 105-110° C. for 20-30 minutes residencetime at 15-17% moisture by weight that a meal can be produced with aprotein digestibility as measured by PDI of 30-45%, substantiallygreater than the PDI of 20-30% presently known in the trade.

It has also been discovered by analytical testing that if the spentmaterial is subjected to a temperature of 105-110° C. for 20-30 minutesresidence time at 15-17% moisture by weight that a meal can be producedwith an ANF as measured by urease delta pH of less than 0.15.

In one exemplary embodiment of the invention, superheated hexane vaporsare used to evaporate solvent from the spent material at the beginningof the DT apparatus and process. This method allows a substantialreduction in solvent content in the spent material without increasingthe spent material moisture or temperature. The result is less solventproceeding to the uppermost CC tray which in turn requires lesscondensation of superheated water vapors into the spent material, whichin turn allows the moisture to be controlled in the spent material inthe ideal range of 15-17% by weight. Under the conditions of 15-17%moisture by weight, 105-110° C. temperature and 20-30 minutes residencetime, ANF is still adequately deactivated while protein digestibility issubstantially improved.

A substantial downside of higher residual solvent in the finished mealproduct was realized when reducing the required amount of superheatedwater vapor condensing into the spent material. This superheated watervapor was also acting as the stripping medium to reduce the residualsolvent content in the meal to less than 500 parts per million (ppm),and preferably less than 250 ppm solvent by weight. The flux rate of therising superheated water vapor fell below the lower threshold of 300kg/hr/m² flux rate required to adequately strip the solvent from thespent material.

In another exemplary embodiment of this invention, water vapor isrecycled from below the upper CC tray to above the SP tray of the DT.The recycled water vapor in combination with the reduced rate of risingwater vapor introduced at the SP tray enables the flux rate through thespent material to be increased over the minimum acceptable flux rate of300 kg/hr/m². Under the conditions of 15-17% moisture by weight,105-110° C. temperature, 20-30 minutes residence time, and a superheatedwater vapor flux rate of greater than 300 kg/hr/m², meal can be producedwith less than 500 parts per million (ppm), and ideally less than 250ppm solvent by weight.

In summary, the ideal conditions of 15-17% moisture by weight, 105-110°C. temperature, 20-30 minutes residence time, and a superheated watervapor flux rate of greater than 300 kg/hr/m² can all be simultaneouslyachieved. As above noted, one aspect of the invention pertains tosuperheated solvent recycle at the initial stage of the DT apparatus andprocess. Another aspect of the invention involves superheated watervapor recycle at the final stage of the DT apparatus and process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating data gathered from a benchtop scale DTshowing the impact of PDI in meal at varying levels of moisture;

FIG. 2 is a graph illustrating data gathered from a benchtop scale DTshowing the impact of ANF in meal at varying levels of moisture; and

FIG. 3 is a cross-section drawing of one embodiment of the improved DTapparatus wherein both a superheated solvent vapor recycle system andsuperheated water vapor recycle system are employed.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

With attention to FIG. 3, an apparatus is shown having superheatedsolvent vapor recycle and superheated water vapor recycle systems. Theartisan will appreciate that either of these systems may be used byitself, or if desired, the systems may be conjointly utilized. As shown,the apparatus comprises a vertical cylindrical housing (1) with an inletfor the spent material at the top of said housing (2) and an outlet forthe desolventized, toasted meal at the bottom of said housing (3). Thespent material is stirred above internal trays (4,5,6,7) by agitators(8) connected to a rotating vertical shaft (9). The trays are seriallyarranged from an upstream location to a downstream location within theinterior of the housing. The spent material passes from an upstreamdirection to a downstream direction from tray to tray by means of avariable speed rotary valve (10), which also controls the spent materiallayer depth on each tray.

The first, top, upstream tray (4) is an enclosed steam-heated tray withapertures therein which allow superheated solvent vapors from the traybelow to rise through the spent material layer supported above. Solventis primarily evaporated from the spent material on this tray by therising solvent vapor losing its superheat, and secondarily by transferof heat from the steam-heated tray surface. The second tray (5) is anenclosed steam-heated tray with no apertures. Solvent is evaporated fromthe spent material on this tray by transfer of heat from thesteam-heated tray surface. This tray (5) forms a vapor seal from thetray below. These two trays (4,5) act as a pair for injection ofsuperheated solvent vapor into the spent material. This apparatus canhave as little as one and as many as four pairs of these trays (4,5) asrequired to supply sufficient superheated solvent to evaporate adequatesolvent from the spent material prior to the spent material passing tothe next zone of the DT apparatus.

The fifth, sixth and seventh trays (6) are enclosed steam-heated trayswith apertures which allow superheated water vapors from the tray belowto rise through the spent material layer supported above. Solvent isprimarily evaporated from the spent material on these trays by therising water vapor condensing into the spent material, and secondarilyby transfer of heat from the steam-heated tray surface.

The final or most downstream tray shown (7) is an enclosed chamber witha plurality of small apertures which allow superheated water vapor inthe form of low pressure steam to enter the apparatus and pass throughthe spent material layer supported above.

Solvent vapor with a slight amount of water vapor exits the sidewall ofthe apparatus as shown at exits (12) and passes to a vapor scrubbingdevice (13) to remove meal dust before this vapor passes on to heatrecovery within the overall solvent extraction process (14).

In one aspect of the invention, the apparatus is equipped with asuperheated solvent vapor recycle system. A portion of the solvent vaporwith slight water vapor exiting the vapor scrubbing device (13) ispulled into a spark-proof blower (15). The spark-proof blower increasesthe pressure and pushes the vapors through the tubes of a shell and tubeheat exchanger (16). Steam on the shell side of this heat exchanger isused to superheat the mostly solvent vapors passing through. The mostlysuperheated solvent vapors enter the sidewall of the vessel, between thepairs of trays (4,5) as shown at 17. These superheated vapors then risethrough the apertures in the tray above, providing heat to evaporatesolvent at low temperature. The cool, evaporated vapors exit thesidewall of the vessel (12) and pass to the vapor scrubbing device (13),completing the recycle loop.

In another aspect of the invention, the apparatus is equipped with asuperheated water vapor recycle system. A portion of the superheatedwater vapor with slight solvent vapor is pulled from the sidewall of thevessel as shown at (18) under the upper countercurrent tray (6) by aspark-proof blower or steam ejector apparatus (19) which in turn passesthe vapor through a steam-heated shell and tube heat exchanger (20) tooptionally add superheat and then returns this vapor back through thesidewall of the vessel into the headspace above the meal of the lowesttray (7) as shown at (21). This return of vapor increases the flux rateof superheated water vapor through the layers of spent materialsupported on the trays (6) to improve stripping of solvent from thespent material. Water vapors (22) from the solvent extraction processmay also be introduced into the superheated water vapor loop for heatrecovery purposes by pushing such vapors with a steam ejector (23).

In certain forms and embodiments, the invention is directed toward anapparatus for processing spent material from a solvent extractionprocess wherein spent material enters the top of a vertical cylindricalvessel (1) with trays (4,5,6,7) disposed horizontally therein whereinspent material is conveyed around such trays by stirrers (8) that areconnected to a vertical rotating champ (9) at the center of the vesseland such spent material is conveyed from tray to tray by rotary airlocks(10). The spent material is subjected to a combination of direct contactby superheated hexane, direct contact by superheated water vapor, andheat transferred from the trays (4,5,6) to enable the solvent to beevaporated and the spent material adequately toasted.

In accordance with another aspect of the invention, a pair of steamheated trays (4,5) is provided in the apparatus wherein the bottom trayacts as a vapor seal and the top tray acts as a superheated solventvapor sparging device to enable significant solvent to be evaporatedfrom spent material with a minimum rise in temperature or moisture.

In one exemplary embodiment, a superheated solvent vapor recycle loop isprovided wherein primary solvent vapors, greater than 90% solvent vaporand less than 10% water vapor exit the apparatus and are returned fromthe vapor scrubber (13) to a vapor tight, spark proof blower (15)wherein such vapors are superheated in a steam-heated shell and tubeheat exchanger (16) and then introduced through the sidewall of thevessel as shown at (17) between the pair of steam heated trays asdefined above which serve as a combination vapor seal and superheatedsolvent vapor sparging device.

The solvent vapors rise through the spent material layer supported onthe upper tray (4) evaporating solvent and such solvent exits thesidewall at the upstream end of the vessel as shown at the upstream exitand proceeds to a vapor scrubber (13) to remove meal dust. The contentof the residual solvent in the spent material after the superheatedsolvent vapor recycle loop through water vapor condensation in uppermostdeep meal layer tray (6) increases the meal moisture to 15-17% moisture,more ideally 15.5-16.5% moisture, and most preferably 16% moisture.Further, the desolventized toasted spent material exiting the apparatushas an ANF as measured by urease delta pH of 0.05 to 0.20, and moreideally, 0.010 to 0.015.

In further exemplary embodiments, the dissolved, toasted spent materialexiting the apparatus has a protein solubility as measured by PDIexceeding 30% and more ideally exceeding 35%, and most preferentiallyexceeding 40%.

In another aspect of the invention, a superheated water vapor recycleloop is provided wherein primary water vapors, greater than 95% watervapor and less than 5% solvent vapor, are pulled through the sidewall ofthe vessel at the head space under the first tray as shown at (18) to avapor type, spark proof blower or steam ejector 19 wherein such vaporsare superheated in a steam heated shell and tube heat exchanger (20) andthen reintroduced through the sidewall of the vessel in the head spaceover the last tray with deep layer material as shown at the arrow 21.Such vapors rise through the tray (6) and spent material supported aboveto strip out solvent. The recycled vapor flow is adjusted to obtain aflux rate through the spent material layer supported by the tray (6) of300-700 kg/hr/m², and more ideally 400-600 kg/hr/m², and most ideally500 kg/hr/m².

In other embodiments of the invention, the desolventized toasted spentmaterial existing the apparatus (3) has a residual solvent content ofless than 500 ppm, and more ideally less than 300 ppm, and most ideallyless than 200 ppm. In another aspect of the invention, any water vaporsource from the solvent extraction process can have its heat recoveredby using a steam ejector 23 or the like to push the vapors into thesuperheated water vapor recycle loop.

While this invention has been described with reference to variousillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiments herein shown and described, as well as other embodiments ofthe invention will be apparent to persons skilled in the art uponreference to this description. It is therefore contemplated that theappended claims will cover any such modifications or embodiments asfollowed in the true scope of the invention.

What is claimed is:
 1. Apparatus for processing spent material havingsolids, residual moisture, and solvent therein resulting from a solventextraction process, said apparatus comprising a housing having aplurality of spaced apart trays therein serially disposed in saidhousing and arranged along an upstream to downstream direction, meansfor conveying said spent material from tray to tray in said apparatusfrom an upstream entry to a downstream exit, means for directlycontacting said spent material with heated water vapor as said spentmaterial is conveyed through at least a portion of said housing, meansfor contacting said spent material with volatile solvent evaporatingfrom said spent material, heating means cooperating with some of saidplurality of trays for indirectly heating said spent material by contactof said spent material with heated surfaces of some of said plurality oftrays, and a solvent recycling system including solvent recycle conduitmeans for providing removal of said solvent from said housing at asolvent exit and returning said removed solvent to said housing at asolvent entry.
 2. Apparatus as recited in claim 1 further comprising asuperheater operatively associated with said solvent recycling systemconduit means to superheat said removed solvent so that said spentmaterial is contacted by superheated solvent.
 3. Apparatus as recited inclaim 2 further comprising a vapor scrubber and blower operativelyconnected to said solvent recycle conduit means.
 4. Apparatus as recitedin claim 3 wherein said superheater comprises a shell and tube heatexchanger.
 5. Apparatus as recited in claim 2 wherein said plurality oftrays includes a pair of adjacent steam heated trays disposed in anupstream to downstream direction and wherein said pair of trays acts asa vapor seal and said entry location for said removed solvent isprovided intermediate said pair of trays.
 6. Apparatus as recited inclaim 1 further comprising a water vapor recycling system includingwater vapor recycle conduit means for providing removal of said watervapor from said housing at a water vapor recycle exit and returningwater vapor to said housing at a water vapor recycle entry.
 7. Apparatusas recited in claim 6 further comprising a steam ejector or bloweroperatively associated with said vapor recycle conduit means and whereinsaid water vapor recycle entry is located downstream from said watervapor recycle exit.
 8. Apparatus for processing spent material havingsolids, residual moisture, and solvent therein resulting from a solventextraction process, said apparatus comprising a housing having aplurality of spaced apart trays therein serially disposed in saidhousing and arranged along an upstream to downstream direction, meansfor conveying said spent material from tray to tray in said apparatusfrom an upstream entry to a downstream exit, means for directlycontacting said spent material with heated water vapor as said spentmaterial is conveyed through at least a portion of said housing, meansfor contacting said spent material with volatile solvent evaporatingfrom said spent material, heating means cooperating with some of saidplurality of trays for indirectly heating said spent material by contactof said spent material with heated surfaces of some of said plurality oftrays, and a water vapor recycling system including water vapor recycleconduit means for providing removal of said water vapor from saidhousing at a water vapor exit and returning water vapor to said housingat a water vapor recycle entry.
 9. Apparatus as recited in claim 8further comprising a superheater in operative association with saidwater vapor recycle conduit means.
 10. Method for processing spentmaterial having solids, residual moisture, and solvent therein resultingfrom a precursor solvent extraction process, said method comprisingconveying said spent material through a housing from an upstream entryto downstream exit, contacting said spent material with heated watervapor in said housing and volatile solvent formed in said housing, saidmethod further comprising recycling some of said volatile solvent, saidvolatile recycling comprising providing a vapor solvent recycle exit insaid housing, and transporting recycled solvent vapor from said vaporsolvent recycle exit to a vapor solvent entry in said housing. 11.Method as recited in claim 10 further comprising superheating saidrecycled vapor solvent.
 12. Method as recited in claim 11 wherein saidspent material exiting said downstream exit has a moisture content ofabout 15-17% moisture.
 13. Method as recited in claim 12 wherein saidspent material exiting said downstream exit has a moisture content ofbetween about 15.5-16.5% moisture.
 14. Method as recited in claim 13wherein said spent material exiting said downstream exit has a moisturecontent of about 16% moisture.
 15. Method as recited in claim 10 whereinsaid spent material at said downstream exit has an ANF as measured byurease delta pH of 0.05-0.20.
 16. Method as recited in claim 15 whereinsaid ANF as measured by urease delta pH is 0.10-0.15.
 17. Method asrecited in claim 16 wherein said spent material at said downstream exithas a protein solubility as measured by PDI exceeding 30%.
 18. Method asrecited in claim 17 wherein said spent material at said downstream exithas a protein solubility as measured by PDI exceeding 40%.
 19. Method asrecited in claim 10 further comprising recycling said heated watervapor, said heated water vapor recycling comprising providing a heatedwater vapor recycle exit in said housing and passing recycled heatedwater vapor from said heated water vapor recycle exit to a heated watervapor entry.
 20. Method as recited in claim 19 further comprisingsuperheating said recycled heated water vapor.
 21. Method as recited inclaim 20 comprising adjusting a flow rate of said recycled heated watervapor to result in a water vapor flow rate through said spent materialin said housing of between about 300-700 kg/hr/m².
 22. Method as recitedin claim 21 wherein said spent material exiting said downstream exit hasa residual solvent content of less than 500 ppm.
 23. Method forprocessing spent material having solids, residual moisture, and solventtherein resulting from a precursor solvent extraction process, saidmethod comprising conveying said spent material through a housing froman upstream entry to downstream exit, contacting said spent materialwith heated water vapor in said housing and volatile solvent formed insaid housing, said method further comprising recycling some of saidheated water vapor, said recycling of said water vapor comprisingproviding a heated water vapor recycle exit in said housing and passingrecycled heated water vapor from said heated water vapor recycle exit toa heated water vapor entry in said housing.
 24. Method as recited inclaim 23 further comprising superheating said recycled heated watervapor.
 25. Method as recited in claim 23 further comprising addition ofrecovered water vapor from a precursor solvent extraction process intosaid recycled heated water vapor.